This invention relates to electrostatic separation processes and in particular to a method and apparatus for repeatedly disbursing and coalescing a relatively polar fluid in a relatively non-polar fluid by passing a stream of the polar fluid in counterflow to the direction of flow of the non-polar fluid through an electric field of varying electric field gradient established by a plurality of composite electrodes.
Separation processes utilizing high voltage electric fields include solid-solid separation in the benefaction of ores, solid or liquid separation from a dielectric liquid, and the use of electrostatic precipitators to remove solid or liquid particles from exhaust gases and air conditioning systems. The electrostatic field utilized in these separation processes is generated by applying an alternating current voltage, a direct current voltage, or some combination thereof, to a grid within the process flow.
When the liquid-liquid separation removes water from an oil-water emulsion in an electrostatic dehydrator, the oil being a non-polar fluid acts as a dielectric and water droplets being polar are coalesced. Coalescense occurs when small water droplets collide and unite to form larger water droplets. Water droplets are coalesced by establishing an electric field between electrode elements and passing the oil-water emulsion through the electric field. Since water is slightly polar, the droplets become further polarized by the electric field. The polarized droplets are attracted to each other and move into coalescense with each other. The larger droplets gravitate to the bottom of the dehydrator where the water is removed. The dehydrated oil moves upward and is removed from the upper portion of the dehydrator.
Small water droplets have a low settling velocity and, therefore, gravitate slowly or are carried along with the through flow. It is desirable to cause small water droplets to coalesce to form larger water droplets, since the larger water droplets gravitate more readily. Small water droplets, however, are more difficult to move through oil and, therefore, require a greater electric field gradient to coalesce. Specifically, to coalesce small water droplets, an intense electric field gradient such as is achieved when a high voltage is applied to electrode elements is required.
Typical desalting requirements are to reduce the salt or brine content of oil well production to one pound of salt per 1,000 barrels of oil. The salt is normally contained within the brine produced in the oil well production. Removing the water removes most of the salt. However, some very small brine droplets remain with the dehydrated oil which can result in an unacceptable level of salt as high as 20 to 30 pounds per 1,000 barrels of oil. Therefore, fresh water, or less salty water is added to contact and dilute the brine remaining in the dehydrated oil well production followed by another stage of dehydration. If the water content is reduced to the same water content of the emulsion after the initial stage of dehydration, the salt content of the dehydrated oil after the second stage of dehydration is also lowered.
Electrodes in prior art dehydrators have been constructed of electrically conductive materials, typically metals, which instantly transport the applied charge to all surfaces of the electrode system, limited only by the power source supplying energy thereto. When these systems are operating without arcing, very little energy is required to sustain the electric field between electrode elements. However, when arcing occurs, a large amount of energy is consumed. Furthermore, even though the arcing occurs at only one location on a highly conductive electrode plate, the electric field collapses over the entire electrode, thereby terminating the dehydration process until the arcing is quenched. To rapidly detect the existence of arcing, quench the arcing and restore the potential on the electrodes, thereby re-establishing the electric field gradient, elaborate control systems have been utilized, such as the voltage control system disclosed in U.S. Pat. No. 4,400,253.
Further disadvantages of the prior art metal electrodes resides in that the full potential applied to the electrode, due to the conductivity of the electrode plates, exists on all edges, corner and points of the electrodes, as well as on electrode support apparatus. This results in the generation of many distorted fields and point charges. Distortions in the electric field and point charges produce high electric field gradients which limit aqueous droplet growth and cause droplet dispersion. When these dispersal mechanisms exist in areas where the process fluids are exiting the electric field, the aqueous droplets generated are not coalesced and, therefore, are carried out with the processed organic stream, such as dehydrated oil.
The more conventional forms of desalting, both for field and refinery processes, utilizes the addition of dilution water, either fresh water or less saline water, to once dehydrated crude oil emulsions. The emulsion, along with the dilution water, is then flowed concurrently through a mixer to increase the probability of the disbursed brine contacting and coalescing with the dilution water. The crude oil stream is then dehydrated again. Removal of the diluted brine is not complete and the salt removed is limited to the fraction of dilution water volume times the mixing efficiency. Frequently, multiple stages of desalting are required to meet salt specifications. Such a multiple stage dehydrator in a single vessel is disclosed in U.S. Pat. No. 4,149,958.
Yet another multiple stage dehydrator is disclosed in U.S. Pat. No. 4,308,127 which teaches passing an emulsion through a series of electric fields to coalesce a polar disbursed phase with the final electrostatic field decreasing by virtue of uniformly charged electrodes physically diverging. The emulsion from which almost all of the polar disbursed phase has been removed is passed through the decreasing electric field as the last step in coalescing the polar disbursed phase.
Application Ser. No. 385,349 assigned to the same assignee as the present application, the disclosure of which is hereby incorporated by reference, discloses a countercurrent dilution water flow system coupled with electrostatically mixing the dilution water with the brine inherent in oil well production. An electrostatic mixer-separator having a plurality of uniformly spaced, substantially parallel conductive electrode plates in which the voltage applied to the electrode plates is modulated becomes the equivalent of an efficient multi-stage mixer/coalescer/separator. The modulated power supply is employed to repetitively, sequentially vary the electric field gradient between adjacent electrode plates in a predetermined manner to achieve periods of increasing electric field gradient for disbursing the dilution water, sustained high electric field gradient for mixing the disbursed dilution water with the brine, thereby increasing the probability of contacting and diluting the brine, followed by reducing electric field gradient for maximum coalescing of the diluted brine. The modulating controller has the capability to detect arcing, and in response thereto, reduces the voltage applied to the electrodes for a sufficient time to quench the arcing then restore the voltage applied to the electrodes to reestablish the electric fields therebetween. During the time of arcing and reduced voltage, the disbursing, mixing and coalescing process ceases. Although the countercurrent dilution water system described above is superior to previous dehydration systems, it is still sensitive to high inlet brine concentrations or high dilution water rates.
What is needed is a method and apparatus for separating brine from an oil emulsion in which arcing will not occur, which would be free of distorted fields and point charges, and would subject the oil well production to an increasing electric field intensity to coalesce and remove relatively large droplets, followed by a high electric field intensity to coalesce small water droplets followed by a decreasing electric field to reduce the shear forces to which the coalesced water droplets are exposed in the exit area to minimize droplet carryover. Such a method and apparatus would further have a counterflow of dilution water which is disbursed and mixed with the emulsion in the high electric field, thereby increasing the possibility of contacting and diluting the remaining brine followed by coalescense for removal.